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Earlier, the attacker, driving a vehicle on nearby Westminster Bridge, ran down several people.

Mark Rowley, head of counterterrorism efforts for London's Metropolitan Police Service, told reporters late Wednesday that Parliament is no longer on lockdown and will meet as usual Thursday, as promised earlier by Prime Minister Theresa May.

"We think we know who the shooter was," Rowley said, confirming that police believe the shooter was inspired by international terrorist attacks. He said authorities are not ready to give a name and asked reporters not to speculate on the attacker's identity while the investigation continues. He also declined to say whether foreign nationals were wounded or killed in the attack until Britain has contacted their home countries.

Rowley identified the security officer who died in the attack as 48-year-old Keith Palmer, a husband and father who had served in British law enforcement for 15 years.

He said overall five people, including the shooter, died in the attack. He said around 40 were injured, including two police officers who are in serious condition. He said the investigation will continue through the night as security officers comb a wide area for clues.

Rowley also said the international threat assessment in Britain will not change, but officers will be deployed more heavily across the nation, especially in highly crowded areas, to keep people safe.

Britain's Prime Minister Theresa May gives a media statement outside 10 Downing street in London, March 22, 2017, following a terror attack in the Westminster area of London earlier Wednesday.

British Prime Minister Theresa May addressed the public on television late Wednesday, calling the attack "sick and depraved." She vowed that it would not affect the daily routines of people living and working in the heart of London, where the attack took place.

She also commended the security officers who responded to the attack, saying the loss of one of their own members made their work reassuring the public "all the more remarkable."

May said any attempt to intimidate the British people through violence and terror is doomed to failure. She also said Parliament will meet on Thursday as usual.

"We will all move forward together," she said, "never giving in to terror, and never allowing the voices of hate to drive us."

Parliament was placed on lockdown after an attacker stabbed a police officer before being shot by other officers on the parliament grounds. The injured officer later died of his injuries. At least two people were killed and eight others injured when a vehicle struck several people on the nearby Westminster bridge.

A senior police officer told VOA that they believe only one assailant was involved in what appears to have been "a three-staged attack."

It began with an SUV being driven over Westminster Bridge right by the House of Commons. The SUV mounted the sidewalk and struck several pedestrians.

A woman lies injured after a shooting incident on Westminster Bridge in London, March 22, 2017.

Attacker rammed pedestrians

According to police sources, the vehicle struck some other pedestrians at the perimeter fence near the gates at Old Palace Yard.

"The attacker then rushed the gates and struggled with a police guard who tried to stop him. The assailant stabbed him several times," the senior police officer said. “Other officers shot the attacker."

Eyewitness accounts

The gunfire was heard at 2:38 p.m. London time inside the House of Commons as lawmakers were debating legislation on pension reform. Eyewitnesses say about half-a-dozen shots were fired.

"It all happened within a minute," witness Tawhid Tanim told VOA. "I came out of where I work and saw a car had pulled up and I heard I just heard bang bang bang and people running everywhere."

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In Washington, the White House said President Donald Trump has spoken with British Prime Minister Theresa May and has been been briefed on the situation in London.

"We obviously condemn today’s attack in Westminster, which the United Kingdom is treating as an act of terrorism, and we applaud the quick response that the British police and their first responders made to the situation," spokesman Sean Spicer said.

The U.S. Department of Homeland Security said it is in close contact with British authorities but that the U.S. domestic security posture remains unchanged.

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As the attack unfolded British Prime Minister May was in a voting lobby adjacent to the chamber of the House of Commons. Her plains-clothes bodyguards drew their weapons and one officer put his arm around May as they quickly escorted her out of the House of Commons, according to Andrew Bridgen, a Conservative lawmaker.

Armed police have cordoned off streets within a block of the parliament building and the Westminster bridge has been shut down. Members of Parliament, who were in session when the attacks began, have been evacuated.

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If confirmed as a terrorist attack, it would be the first in Britain since May 2013 when an off-duty British Army soldier, Lee Rigby, was hacked to death on a London street by a self-proclaimed jihadist. It is the worst attack at the House of Commons since the IRA bombing in 1979, when Conservative politician Airey Neave, a confidant of Margaret Thatcher, was killed.